Emissions regulations for internal combustion engines have become more stringent over recent years. Environmental concerns have motivated the implementation of stricter emission requirements for internal combustion engines throughout much of the world. Governmental agencies, such as the Environmental Protection Agency (EPA) in the United States, carefully monitor the emission quality of engines and set acceptable emission standards, to which engines must comply. Generally, emission requirements vary according to engine type. Emission tests for compression-ignition (diesel) engines typically monitor the release of diesel particulate matter (PM), nitrogen oxides (NOx), and unburned hydrocarbons (UHC).
Exhaust aftertreatment systems receive and treat exhaust gas generated by an internal combustion engine. Typical exhaust aftertreatment systems include any of various components configured to reduce the level of regulated exhaust emissions present in the exhaust gas. For example, some exhaust aftertreatment systems for diesel powered internal combustion engines include various components, such as a diesel oxidation catalyst (DOC), particulate matter filter or diesel particulate filter (DPF), and a selective catalytic reduction (SCR) catalyst. In some exhaust aftertreatment systems, exhaust gas first passes through the diesel oxidation catalyst, then passes through the diesel particulate filter, and subsequently passes through the SCR catalyst.
A common DPF comprises a porous ceramic matrix with parallel passageways through which exhaust gas passes. Particulate matter subsequently accumulates on the surface of the filter, creating a buildup which must eventually be removed to prevent obstruction of the exhaust gas flow. Common forms of particulate matter are ash and soot. Ash, typically a residue of burnt engine oil, is substantially incombustible and builds slowly within the filter. Soot, chiefly composed of carbon, results from incomplete combustion of fuel and generally comprises a large percentage of particulate matter buildup. Various conditions, including, but not limited to, engine operating conditions, duty cycles, hours of operation, mileage, driving style, terrain, etc., affect the rate at which particulate matter accumulates within a diesel particulate filter.
Accumulation of particulate matter typically causes backpressure within the exhaust system. Excessive backpressure on the engine can degrade engine performance. Particulate matter, in general, oxidizes in the presence of NO2 at modest temperatures, or in the presence of oxygen at higher temperatures. If too much particulate matter has accumulated when oxidation begins, the oxidation rate may get high enough to cause an uncontrolled temperature excursion. The resulting heat can destroy the filter and damage surrounding structures. Recovery can be an expensive process.
To prevent potentially damaging situations, accumulated particulate matter is commonly oxidized and removed in a controlled regeneration process before excessive levels have accumulated. To oxidize the accumulated particulate matter, filter temperatures generally must exceed the temperatures typically reached at the filter inlet. Consequently, additional methods to initiate regeneration of a diesel particulate filter may be used. In one method, a reactant, such as diesel fuel, is introduced into an exhaust after-treatment system to increase the temperature of the filter and initiate oxidation of particulate buildup. A filter regeneration event occurs when substantial amounts of soot are consumed on the particulate filter.
A controlled regeneration can be initiated by the engine's control system for example when a predetermined amount of particulate has accumulated on the filter, when a predetermined time of engine operation has passed, when the backpressure exceeds a predetermined value, or when the vehicle has driven a predetermined number of miles. Oxidation from oxygen (O2) generally occurs on the filter at temperatures above about 400 degrees centigrade (C), while oxidation from nitric oxides (NO2), sometimes referred to herein as noxidation, generally occurs at temperatures between about 250 C and 400 C. Controlled regeneration typically consists of driving the filter temperature up to O2 oxidation temperature levels for a predetermined time period such that oxidation of soot accumulated on the filter takes place. The temperature of the particulate filter is dependent upon the temperature of the exhaust gas entering the particulate filter. Accordingly, the temperature of the exhaust must be carefully managed to ensure that a desired particulate filter inlet exhaust temperature is accurately and efficiently reached and maintained for a desired duration to achieve a controlled regeneration event that produces desired results.
The SCR catalyst reduces the amount of nitrogen oxides (NOx) present in the exhaust gas. Generally, the SCR catalyst is configured to reduce NOx into constituents, such as N2 and H2O, in the presence of ammonia (NH3). Because ammonia is not a natural byproduct of the combustion process, it must be artificially introduced into the exhaust gas prior to the exhaust gas entering the SCR catalyst. Typically, ammonia is not directly injected into the exhaust gas due to logistical and, in turn, economic considerations. Accordingly, conventional systems are designed to inject a reductant (e.g., diesel exhaust fluid (DEF), ammonia, metal chloride salt, etc.) into the exhaust gas, which is capable of decomposing into gaseous ammonia in the presence of exhaust gas under certain conditions. The reductant commonly used by conventional exhaust aftertreatment systems is DEF, which is a urea-water solution.
Generally, the decomposition of reductant into gaseous ammonia occupies three stages. First, the reductant mixes with exhaust gas and water is removed from the reductant through a vaporization process. Second, the temperature of the exhaust causes a thermolysis-induced phase change in the reductant and decomposition of the reductant into isocyanic acid (HNCO) and NH3. Third, the isocyanic acid reacts with water in a hydrolysis process to decompose into ammonia and carbon dioxide (CO2). The gaseous ammonia is then introduced at the inlet face of the SCR catalyst, flows through the catalyst, and is consumed in the NOx reduction process. Any unconsumed ammonia exiting the SCR system can be reduced to N2 and other benign components using an ammonia oxidation catalyst.
SCR systems typically include a reductant source and a reductant injector or doser coupled to the source and positioned upstream of the SCR catalyst. The reductant injector injects reductant into a decomposition space or tube through which an exhaust gas stream flows. Upon injection into the exhaust gas stream, the injected reductant spray is heated by the exhaust gas stream to trigger the decomposition of reductant into ammonia. As the reductant and exhaust gas mixture flows through the decomposition tube, the reductant further mixes with the exhaust gas before entering an the SCR catalyst. Ideally, reductant is sufficiently decomposed and mixed with the exhaust gas prior to entering the SCR catalyst to provide an adequately uniform distribution of ammonia at the inlet face of the SCR catalyst.
Some prior art exhaust aftertreatment systems, however, do not provide adequate decomposition and mixing of injected reductant. Often, conventional systems cause low temperature regions within the decomposition tube. Further, during low load operating conditions, the temperature of the exhaust gas may be relatively low. Low temperature exhaust gas and regions within the exhaust gas may result in inadequate mixing or decomposition, which may lead to the formation of solid reductant-based or UHC deposits on the inner walls of the decomposition tube and reductant-based injector. Solid reductant-based deposits include the solid byproducts from incomplete decomposition of urea, such as biuret, cyanuric acid, ammelide, and ammeline, and UHC deposits include UHC adsorbed by the SCR catalyst. Additionally, inadequate mixing may result in a low reductant vapor uniformity index, which can lead to uneven distribution of the reductant across the SCR catalyst surface, lower NOx conversion efficiency, and other shortcomings. The formation of solid reductant-based deposits within the decomposition tube typically results in a lower amount of ammonia concentration and a lower ammonia distribution uniformity index at the inlet face of the SCR catalyst, which can degrade the performance and control of the SCR catalyst. Additionally, solid reductant-based deposits in the decomposition tube can induce exhaust backpressure within the exhaust aftertreatment system, which can adversely impact the performance of the engine and exhaust aftertreatment system.
Some conventional systems recognize the negative effect of reductant-based deposit buildup on the SCR catalyst and other SCR components caused by low exhaust temperatures. Such systems employ regeneration techniques to increase the temperature of the exhaust in the SCR system to regenerate the SCR catalyst and remove the reductant-based deposits from the SCR catalyst.
Conventional systems use various strategies for managing the particulate filter and SCR system inlet exhaust temperatures. For example, some systems use a combination of air handling strategies, internal fuel dosing strategies, and external fuel dosing strategies.
The air handling strategies include managing an air intake throttle to regulate the air-to-fuel ratio. Lower air-to-fuel ratios, e.g., richer air/fuel mixtures, typically produce higher engine output exhaust temperatures.
Internal fuel dosing strategies include injecting additional fuel into the compression cylinders. Such in-cylinder injections include pre-injections or fuel injections occurring before a main fuel injection and post-injections or fuel injection occurring after a main fuel injection. Generally, post-injections include heat post-injections and non-heat post-injections. Heat post-injections are injections that participate along with the main fuel injection in the combustion event within the cylinder and occur relatively soon after the main fuel injection. Non-heat post injections are injections that occur later in the duty cycle compared to the heat post-injections and do not participate in the combustion event within the cylinder.
External fuel dosing strategies include injecting fuel into the exhaust gas stream at locations downstream of the engine. Typically, external fuel dosers are positioned in the exhaust aftertreatment system between the engine and a catalytic component, e.g., the DOC. The DOC reduces the number of pollutants in the exhaust gas through an oxidation process prior to the gas entering the particulate filter. The catalyst of the catalytic component must be at a specific temperature for oxidation of the pollutants to occur. The oxidation process heats the exhaust and causes the temperature of the exhaust to increase. In other words, during an oxidation process on the DOC, the DOC outlet exhaust temperature typically is greater than the DOC inlet exhaust temperature. Because fuel in the exhaust participates in the oxidation process, the exhaust temperature differential across the DOC, and thus the DOC outlet exhaust temperature, is largely dependent upon the amount of fuel in the exhaust gas entering the DOC.
Air handling strategies are aimed at controlling engine output exhaust temperatures. Internal fuel dosing strategies affect both engine output exhaust temperatures and DOC outlet exhaust temperatures. Fuel from internal fuel injections not combusted in the combustion event is oxidized in the DOC and increases the DOC outlet exhaust temperature. Similarly, external fuel injections simply add fuel to the exhaust stream, and thus increase the DOC outlet exhaust temperature.
In typical systems, the particulate filter receives exhaust directly from the DOC. Accordingly, the particulate filter inlet exhaust temperature is approximately equal to the DOC outlet exhaust temperature. Therefore, an important tool in achieving a desired particulate filter inlet exhaust temperature is to ensure that the DOC is operating at the proper temperature for oxidation to occur. Because the temperature of the catalytic component is dependent upon the engine output exhaust temperature, many conventional engine systems employ various methods for controlling engine output exhaust temperature. However, such conventional methods can have several shortcomings. For example, for some mid-range engines operating under low load conditions and urban-type driving conditions (e.g., start-stop driving conditions with long periods of idle engine running), the engine output exhaust temperature control methods for such engines may be unable to achieve the engine output exhaust temperatures necessary to reach DOC activation temperatures. Moreover, when the engine is operating under low ambient air temperatures, it may be even more difficult for conventional engine output exhaust temperature control methods to attain adequately high engine output exhaust temperatures. When the DOC activation temperatures are not reached, the particulate filter may not adequately be able to regenerate, possibly resulting in an increase in backpressure from the accumulation of particulate, and a possibility of an uncontrolled regeneration event that may damage the DPF.